Like other coatings, container coatings will inevitably produce some paint defects during the construction process. Different from general coatings , due to the assembly line construction, it needs to be adjusted immediately after the occurrence of paint film defects, and the current production rate generally does not allow the line to be stopped to repair the paint film defects or adjust the paint, so it needs to be used after the paint film defects occur. The fastest and easiest way is to adjust as soon as possible , otherwise it will cause huge economic losses to the container factory, which needs to be considered when designing container coatings . The causes and solutions of several common paint diseases are summarized as follows
1. Hanging
There are usually three reasons for sagging: excessive thickness of the coating due to various reasons, insufficient thixotropy of the coating itself, and problems with the adjustment of the coating and the ambient temperature . Except for zinc-rich paint, container coatings are all thick-film coatings. Higher sag resistance is considered in coating design . But in order to obtain a better surface state, the leveling performance of the coating needs to be better. Therefore , the relationship between uniform film thickness and other properties is the key to solve the problem of sagging when designing the coating formulation . In addition, changes in some construction parameters will tend to increase the thickness of the coating film. Adjusting these process parameters can also solve the sag generated during the construction process.
2. Pinhole
Generally speaking, pinholes are mainly caused by the surface of the paint film being closed too quickly and the underlying solvent is not completely volatilized. Too thick a paint film, too short a standing time, excessive ventilation and too fast heating can cause pinholes. Too many air bubbles mixed during stirring are also prone to pinholes. Another type of pinhole is actually caused by poor leveling of the coating and poor wetting of the substrate. According to different reasons , corresponding measures should be adopted to solve them.
3. Shrinkage cavity
Shrinkage is caused by the difference in surface tension between the coating and the substrate (or the underlying paint). When the surface tension of the coating is too high coating cannot completely wet the substrate, and shrinkage will occur. When the coating has a tendency to shrink, small impurities can also aggravate the shrinkage. Contamination of the base layer and excessive drying can cause cratering. There are many reasons for shrinkage cavities in the production process of containers , and they are also very complicated. In addition to oil, water and dust, oil, wax and additives used in other processes may cause shrinkage cavities. The effective way to solve the shrinkage cavity is to add silicone oil additives , but be careful to dissolve the silicone oil in the solvent before adding it to the paint, and control the amount added. It is worth noting that after adding silicone oil anti-cratering additives, the surface energy of the topcoat will be reduced, which often affects the adhesion .
4. Orange peel
Orange peel is usually a decorative defect during construction, mainly due to poor leveling of the paint. The direct reason is that the viscosity of the paint is too high and it is not easy to level; the spraying pressure is too low, the atomization is not good, etc. The orange peel will affect the gloss and appearance of the paint
5. Whitening
Whitening is a common disadvantage of thermoplastic exterior paint, mainly because the surface temperature of the paint decreases If the surrounding air humidity is too high, water vapor will condense on the surface of the paint film, and part of the water will also enter the paint. The film appears white because it is difficult to dissipate. The other part will be attached to the surface of the paint film and occupy the position on the surface of the paint film. When the water volatilizes, it will cause microscopic unevenness on the surface of the paint film, and form a diffuse reflection of light to appear white in appearance. Some of the whitening phenomenon will become lighter as the water vapor evaporates during the drying process of the coating film, and some are difficult to recover. For less serious whitening, spray a layer of solvent on the surface to "dissolve" the surface of the coating or take away the moisture. Severe white beard sanding and refinishing.
6. Cracking
The cracking immediately after the construction is usually caused by the difference in tension between the coating films. It is easy to see that when different types of coatings are overcoated, in addition to the reason of formula design, the mismatch of drying rate between the inner and outer layers is the main cause of cracking.
7. Poor adhesion
Since the substrate treatment process and coating matching are relatively mature, the probability of poor adhesion of container coatings is not high. Only when the steel plate is seriously polluted by oil stains or there is a large amount of dust and impurities on the surface will there be large-scale shedding. Other cases of reduced adhesion are related to the drying during construction and the treatment of the underlying paint film . See Table 3-4-54 for the common paint film problems and solutions in the container production process.
